This invention relates to active matrix electroluminescent display devices comprising a matrix array of electroluminescent display elements each of which has an associated switching means for controlling the current through the display element.
Matrix display devices employing electroluminescent, light-emitting, display elements are well known. As for the display elements organic thin film electroluminescent elements and light-emitting diodes (LEDs), comprising traditional III-V semiconductor compounds, have been used. In the main, such display devices have been of the passive type in which the electroluminescent display elements are connected between intersecting sets of row and column address lines and addressed in multiplexed fashion. Recent developments in (organic) polymer electroluminescent materials have demonstrated their ability to be used practically for video display purposes and the like. Electroluminescent elements using such materials typically comprise one or more layers of a semiconducting conjugated polymer sandwiched between a pair of (anode and cathode) electrodes, one of which is transparent and the other of which is of a material suitable for injecting holes or electrons into the polymer layer. An example of such is described in an article by D. Braun and A. J. Heeger in Applied Physics Letters 58 (18) p.p. 1982-1984 (May 6, 1991). By suitable choice of the conjugated polymer chain and side chains, it is possible to adjust the bandgap, electron affinity and the ionisation potential of the polymer. An active layer of such a material can be fabricated using a CVD process or simply by a spin-coating technique using a solution of a soluble conjugated polymer. Through these processes, LEDs and displays with large light-emitting surfaces can be produced.
Organic electroluminescent materials offer advantages in that they are very efficient and require relatively low (DC) drive voltages. Moreover, in contrast to conventional LCDs, no backlight is required. In a simple matrix display device, the material is provided between sets of row and column address conductors at their intersections thereby forming a row and column array of electroluminescent display elements. By virtue of the diode-like I-V characteristic of the organic electroluminescent display elements, each element is capable of providing both a display and a switching function enabling multiplexed drive operation. However, when driving this simple matrix arrangement on a conventional row at a time scanning basis, each display element is driven to emit light for only a small fraction of the overall field time, corresponding to a row address period. In the case of an array having N rows for example, each display element can emit light for a period equal to f/N at most where f is the field period. In order then to obtain a desired mean brightness from the display, it is necessary that the peak brightness produced by each element must be at least N times the required mean brightness and the peak display element current will be at least N times the mean current. The resulting high peak currents cause problems, notably with the more rapid degradation of the display element lifetime and with voltage drops caused along the row address conductors.
One solution to these problems is to incorporate the display elements into an active matrix whereby each display element has an associated switch means which is operable to supply a drive current to the display element so as to maintain its light output for a significantly longer period than the row address period. Thus, for example, each display element circuit is loaded with an analogue (display data) drive signal once per field period in a respective row address period which drive signal is stored and is effective to maintain a required drive current through the display element for a field period until the row of display elements concerned is next addressed. This reduces the peak brightness and the peak current required by each display element by a factor of approximately N for a display with N rows. An example of such an active matrix addressed electroluminescent display device is described in EP-A-0717446. The conventional kind of active matrix circuitry used in LCDs cannot be used with electroluminescent display elements as such display elements need to continuously pass current in order to generate light whereas the LC display elements are capacitive and therefore take virtually no current and allow the drive signal voltage to be stored in the capacitance for the whole field period. In the aforementioned publication, each switch means comprises two TFTs (thin film transistors) and a storage capacitor. The anode of the display element is connected to the drain of the second TFT and the first TFT is connected to the gate of the second TFT which is connected also to one side of the capacitor. During a row address period, the first TFT is turned on by means of a row selection (gating) signal and a drive (data) signal is transferred via this TFT to the capacitor. After the removal of the selection signal the first TFT turns off and the voltage stored on the capacitor, constituting a gate voltage for the second TFT, is responsible for operation of the second TFT which is arranged to deliver electrical current to the display element. The gate of the first TFT is connected to a gate line (row conductor) common to all display elements in the same row and the source of the first TFT is connected to a source line (column conductor) common to all display elements in the same column. The drain and source electrodes of the second TFT are connected to the anode of the display element and a ground line which extends parallel to the source line and is common to all display elements in the same column. The other side of the capacitor is also connected to this ground line. The active matrix structure is fabricated on a suitable transparent, insulating, support, for example of glass, using thin film deposition and process technology similar to that used in the manufacture of AMLCDs.
With this arrangement, the drive current for the light-emitting diode display element is determined by a voltage applied to the gate of the second TFT. This current therefore depends strongly on the characteristics of that TFT. Variations in threshold voltage, mobility and dimensions of the TFT will produce unwanted variations in the display element current, and hence its light output. Such variations in the second TFTs associated with display elements over the area of the array, or between different arrays, due, for example, to manufacturing processes, lead to non-uniformity of light outputs from the display elements.
It is an object of the present invention to provide an improved active matrix electroluminescent display device.
It is another object of the present invention to provide a display element circuit for an active matrix electroluminescent display device which reduces the effect of variations in the transistor characteristics on the light output of the display elements and hence improves the uniformity of the display.
This objective is achieved in the present invention by making use of the fact that transistors fabricated close together will usually have very similar characteristics.
According to the present invention, there is provided an active matrix electroluminescent display device of the kind described in the opening paragraph which is characterised in that the switching means associated with a display element comprises a current mirror circuit which is operable to sample and store a drive signal that determines the display element drive current and applied during a display element address period and to maintain the display element drive current following the address period, the current mirror circuit comprising a first transistor whose current-carrying electrodes are connected between a supply line and an electrode of the display element, a second transistor to whose gate electrode and first current-carrying electrode the drive signal is applied and whose second current-carrying electrode is connected to the supply line, the gate of the first transistor being connected to the supply line via a storage capacitor and to the gate of the second transistor via a switch device which is operable to connect the gates of the first and second transistors during the address period. The use of a current mirror circuit in this way overcomes the aforementioned problems by ensuring that the currents driving display the elements are not subject to the effects of variations in the characteristics of individual transistors supplying the currents.
In operation of this display element circuit, a drive signal applied to the first current-carrying electrode and the gate electrode of the second transistor during an address period for the display element concerned results in a current flowing through this diode-connected transistor. By virtue of the gate electrodes of the first and second transistors being interconnected during this period by the switch device, this current is then mirrored by the first transistor to produce a drive current flow through the display element proportional to the current through the second transistor and to establish a desired voltage across the storage capacitor which is equivalent to the gate voltage on the two transistors required to produce that current. At the end of the address period the gates of the transistors are disconnected, by operation of the switch device, and the gate voltage stored on the storage capacitance serves to maintain operation of the first transistor and the drive current through the display element, and hence its desired light output, at the set level. Preferably, the characteristics of the first and second transistors forming the current mirror circuit are closely matched as the operation of the circuit is then most effective.
With this arrangement an improvement in the uniformity of light output from the display elements is achieved.
The transistors can conveniently be provided as TFTs and fabricated on a suitable, insulating, substrate. It is envisaged though that the active matrix circuitry of the device may be fabricated using IC technology using a semiconductor substrate and with the upper electrode of the display elements being of transparent material such as ITO.
Preferably, the display elements are arranged in rows and columns, and the switch devices of the current mirror circuits for a row of display elements, which preferably similarly comprise transistors such as TFTs, are connected to a respective, common, row address conductor via which a selection signal for operating the switch devices in that row is supplied, and each row address conductor is arranged to receive a selection signal in turn. The drive signals for the display elements in a column are preferably supplied via a respective column address conductor common to the display elements in the column. Similarly, the supply line is preferably shared by all display elements in the same row or column. A respective supply line may be provided for each row or column of display elements. Alternatively, a supply line could effectively be shared by all display elements in the array using for example lines extending in the column or row direction and connected together at their ends or by using lines extending in both the column and the row directions and connected together in the form of a grid. The approach selected will depend on the technological details for a given design and fabrication process.
For simplicity, a supply line which is associated with, and shared by, a row of display elements may comprise the row address conductor associated with a different, preferably adjacent, row of display elements via which a selection signal is applied to the switch devices of the current mirror circuits of that different row.
The drive signal may be supplied to the second transistor via a further switch device, for example, another transistor connected between the column address conductor and the second transistor, and operable in the case of this further switch device comprising a transistor by the selection signal applied to the row address conductor. However, in the case where the supply line is constituted by an adjacent row conductor the need to provide such a further switch device may be avoided by using an appropriate drive waveform on the adjacent row address conductor to which the first and second transistors are connected which includes, in addition to the selection signal intended for the switch devices of the adjacent row of display elements, a further voltage level at the appropriate time, i.e. during the address period for the row of display elements concerned, which causes the diode-connected second transistor to conduct.
In the case where an adjacent row address conductor is not used as the supply line connected to the first and second transistors, then as the rows of display elements are addressed separately, i.e. one at a time in sequence, it is possible for the second transistor of the current mirror circuit to be shared by, and thus common to, the current mirror circuits of all the display elements in the same column. To this end, this diode-connected second transistor may be connected between the column address conductor and a source of potential corresponding to that of the supply line and the gate of the first transistor connected to the column address conductor through the switch device. As before, the application of a drive signal to the column address conductor generates a current which flows through this transistor and the column address conductor thus has a potential relative to the potential of the supply line equal to the voltage across the transistor. Assuming the switch device of the display element is turned on this voltage is applied to the gate of the first transistor, and the storage capacitor, so that the two transistors form a current mirror as before. This arrangement has the advantage that the number of transistors required for the display elements of each column is considerably reduced which is not only likely to improve yield but also increase the area available for each display element.